Conventionally, a liquid crystal display device has been widely used for the screen of a word processor and a computer. In recent years, the liquid crystal display device has spread rapidly as a television screen. Many of these liquid crystal display devices adopt TN (Twisted Nematic) mode. However, the TN mode liquid crystal display device has the problems of tendencies to degrade a contrast and to reverse a gradation property when viewed from an oblique direction.
For this reason, in recent years, in light of the improvement in viewing angle property when viewed from an oblique direction, a VA (Vertical Alignment) mode liquid crystal display device has been attracting attention. A liquid crystal cell of the VA mode liquid crystal display device is arranged in combination of a nematic liquid crystal having a negative dielectric anisotropy with a vertical alignment film.
Further, for example, Japanese Patent No. 2947350 (published on Sep. 13, 1999) and Japanese Laid-Open Patent Publication No. 2000-39610 (published on Feb. 8, 2000), as shown in FIGS. 16 and 17, disclose a liquid crystal display device 101 in which a biaxial film 116 is provided between a liquid crystal cell 111 and a polarizing plate 112, and a liquid crystal display device 101a in which a positive uniaxial film 114 and a negative uniaxial film 115 are provided between the liquid crystal cell 111 and the polarizing plate 112 and between the positive uniaxial film 114 and the polarizing plate 112, respectively, in order to optically compensate for the optical anisotropy of the liquid crystal cell 111 when black image is displayed.
With the above arrangement, even though the liquid crystal cell 111 brings a phase difference depending on a polar angle to transmitting light when the liquid crystal cell 111 in which liquid crystal molecules are oriented vertically is viewed from an oblique direction, the phase difference can be compensated by properly setting the respective retardations of the film 116 (films 114 and 115). Therefore, a black display can be performed substantially as in the case when the liquid crystal cell 111 is viewed from a front direction, that is, as in the case where the liquid crystal molecules maintain the polarized state of the transmitting light. As a result of this, it is possible to prevent light leakage, thus enhancing the contrast and suppressing the occurrence of coloring and tone degradation when viewed from an oblique direction.
However, nowadays, under the situation where a liquid crystal display device with a wider viewing angle and higher display quality is expected, there is a demand for the improvement in coloring and tone degradation caused when viewed from the oblique direction. The liquid crystal display device using the films 116 (114 and 115) with the retardation described in Japanese Patent No. 2947350 and Japanese Laid-Open Patent Publication No. 2000-39610, however, is not always satisfactory and still leaves room for improvement.
In view of the above problems, the present invention in a vertical alignment mode liquid crystal display device is attained as a result of investigating the effects of base films of polarizing plates on suitable retardations of films for the suppression of coloring and tone degradation when viewed from an oblique direction. It is therefore an object of the present invention is to surely provide a liquid crystal display device in which the coloring and tone degradation can be suppressed within allowable limits in practical use, while maintaining a contrast at a sufficiently high value in practical use when viewed from an oblique direction.